Apparatus and method for tuning center frequency of a filter

ABSTRACT

A tuning method and a tuning apparatus for tuning a filter are disclosed. The tuning method includes: configuring the filter as a VCO; utilizing the VCO to generate an oscillation signal according to a driving signal; comparing a frequency of the oscillation signal with a reference frequency to generate a comparison result; converting the comparison result into the driving signal in order to establish a feedback mechanism. Therefore, the inner components such as the gm and capacitance inside the VCO are completely tuned when the VCO generates an oscillation signal having a wanted frequency. Since the VCO is inside the filter and the components of the filter and the VCO are similar, the driving signal can be utilized to make the filter operate in a desired center frequency under a well-designed relationship between the frequency of the oscillation signal and the center frequency.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional applicationNo.60/597,333, which was filed on Nov. 23, 2005 and entitled “APPARATUSAND METHOD FOR FILTER FREQUENCY TUNING”.

BACKGROUND

The invention relates to a tuning apparatus and method, and moreparticularly, to an apparatus for tuning a center frequency of a filterand related method thereof.

Filters have been widely used in many applications. In general, theimplementations of the filters can be roughly divided into two types ofstructures, which are discrete-time switch-capacitor filters andcontinuous-time filters (including gm-C filter, MOSFET-C filter, etc.).Because of the limitation of clock frequency, a high-frequency filter isoften implemented by the continuous-time structure. Furthermore, indifferent types of continuous-time filters, the gm-C filter is the mostgeneral.

However, the variation degree of some characteristics, such as cut-offfrequency (center frequency), of the continuous-time filter is oftenlarger than 30% due to the influences of variations in the manufacturingprocess and temperature variations. Therefore, a tuning mechanism shouldbe added into the filter to overcome the process and temperaturevariations such that the frequency response of the filter is notaffected by the influences of the process and temperature variations.

In the gm-C filter structure, the cut-off frequency (center frequency)can be represented by the following equation w_(c)=K*g_(m,u)/C, whereg_(m,u) represents a gm(trans-conductance) value of a trans-conductorper unit, C represents a total capacitance value corresponding to anode, and K is a scaling factor larger than 0. From the above equation,it can be seen that when the center frequency w_(c) deviates from atarget value, the gm value g_(m,u) or the capacitance value C can beadjusted to tune the center frequency w_(c) back to the target value. Inorder to achieve the purpose of tuning the center frequency w_(c),either the gm value g_(m,u) or the capacitance value C can be adjusted.Please note that adjusting the gm value g_(m,u) or the capacitance valueC is equivalent. The spirit of the two adjusting methods is the same.

Taking the method of adjusting the gm value g_(m,u) as an example,please refer to FIG. 1, which shows a conventional tuning structure 100of adjusting the gm value g_(m,u) of a main filter 110. Please note, themain filter 110 is a target filter to be tuned. Furthermore, in FIG. 1,the tuning operation is performed by a PLL (including FD 120 the chargepump 121 and the loop filter 122) cooperating with a VCO 130. Pleasenote, under the tuning structure 110 shown in FIG. 1, the VCO 130 isbetter composed of the same trans-conductor cells as those of the mainfilter 110, the VCO 130 has the same environment (e.g., loading, etc.)as the main filter 110, and the gm value of the trans-conductor circuitsof the VCO 130 and the main filter 110 are controlled by the samecontrol signal Vc. Therefore, if the tracking relationship between theVCO 130 and the main filter 110 is better, the tuning operation of thetuning structure can be more accurate. In other words, when the VCO 130is tuned, the main filter 110 is also tuned because they have similarenvironment. Assume that the center frequency w_(c) of the main filter110 is ideally equal to the K*g_(m,u)/C, and the oscillation frequencyw_(o) of the VCO 130 is equal to N*g_(m,u)/C. When the PD 120 of the PLLis locked to a certain frequency, the control signal Vc is adjusted tochange the gm value g_(m,u) such that the f_(o)=(½π)(N*g_(m,u)/C)=f_(ref). As mentioned previously, the main filter 110 andthe VCO 130 has a good tracking relationship (e.g., they have the samegm value g_(m,u)). Therefore, f_(c)=(½π) (K*g_(m,u)/C)=(K/N) f_(o)=(K/N)f_(ref). Obviously, if the values K, N, and f_(ref) can be selectedproperly, the center frequency of the main filter 110 can be tuned to atarget frequency.

Please refer to FIG. 2, which is a diagram of another conventionaltuning structure 200. Please note, the tuning structure 200 shown inFIG. 2 utilizes a similar concept. The tuning structure 200 utilizessimilar trans-conductor cells to form the master filter 230 (in general,the master filter shown in FIG. 2 has lower levels), and utilizes thecharacteristic of the master filter 230 to perform the tuning tasks. Forexample, a two-level Biquad LPF has a 90 degree phase delay at the pointwhere w_(o)=N*g_(m,u)/C. Therefore, when the signal having the frequencyf_(ref) is inputted into the master filter 230, the entire tuningstructure 200 utilizes the phase detector (PD) 220 to determine whetherthe phase difference is 90 degrees. Additionally, recall as mentionedpreviously, the feedback mechanism is implemented by a PLL including acharge pump 221 and a loop filter 222, the negative feedback mechanismadjusts the control signal Vc to make the f_(o)=(½π)(N*g_(m,u)/C)=f_(ref). From the above-mentioned structure, it can beeasily seen that f_(c)=(½π) (K*g_(m,u)/C)=(K/N) f_(o)=(K/N) f_(ref).Therefore, the tuning structure 200 shown in FIG. 2 can also achieve thesame tuning goal.

The above-mentioned structures both needs a PLL including a PD, a chargepump, and a loop filter. It is known that the PLL occupies a larger areaand as one result, this increases the cost. Please refer to FIG. 3,which is a diagram of another conventional tuning mechanism 300. Thetuning mechanism 300 utilizes a digital circuit 320 to perform anegative feedback control. The entire tuning method shown in FIG. 3 ismore similar to the tuning structure shown in FIG. 1. The differencebetween the tuning structures shown in FIG. 3 and FIG. 1 is that thetuning structure shown in FIG. 3 utilizes a digital circuit 320 (i.e., adigital FD) to compare the frequency f_(ref) with the frequency f_(c)generated by the VCO 330 instead of utilizing a PLL. Thereby, thecomparison result is transformed into a control signal through a DAC 340in order to adjust the frequency f_(c). Similarly, because of thetracking relationship between the VCO 330 and the main filter, when theoscillation signal of the VCO 330 is tuned, the cut-off frequency of themain filter 310 can be tuned successfully.

The influences caused by the process and temperature variations upon thefrequency f_(c), can be alleviated through the above-mentioned tuningmechanisms. Obviously, the above-mentioned tuning mechanisms need eithera VCO or a master filter. Furthermore, either the VCO or the masterfilter is often a two-level system. In addition, in order to make theenvironment similar to the main filter, all dummy devices, dummyloading, and other circuits, which have originally been set up in themain filter, also have to be copied and implemented inside the tuningstructure (e.g., the above-mentioned VCO or master filter) to make theenvironment similar. In most of the applications, the tuning structureoften occupies a huge area larger than 20% of the entire circuit.Therefore, the above-mentioned tuning mechanism consumes a large areaand high cost resulting in an uneconomical solution.

SUMMARY OF THE INVENTION

It is therefore one of the primary objectives of the claimed disclosureto provide a tuning apparatus, to solve the above-mentioned problem.

According to an exemplary embodiment of the claimed disclosure, a tuningapparatus for tuning a filter is disclosed. The filter comprises avoltage-controlled oscillator (VCO) circuit. The tuning apparatuscomprises: an enabling circuit, electrically connected to the filter,for controlling the filter to enter a tuning mode by disabling theentire filter except for the VCO to thereby allow the VCO to generate anoscillation signal according to a driving signal; and for controllingthe filter to enter a normal mode from the tuning mode by enabling theentire filter to operate according to a driving signal when a frequencyof the oscillation signal is equal to a reference frequency; a frequencydetector, electrically connected to the enabling circuit and the VCO,for comparing a frequency of the oscillation signal outputted by the VCOwith the reference frequency to generate a comparison result; and acontrolling circuit, electrically connected to the frequency detectorand the filter, for adjusting the driving signal according to thecomparison result in the tuning mode, obtaining the driving signal inthe tuning mode when the frequency of the oscillation signal is equal tothe reference frequency, and outputting the driving signal to the filterin the normal mode.

According to an exemplary embodiment of the claimed disclosure, a tuningmethod for tuning a filter is disclosed. The filter comprises avoltage-controlled oscillator (VCO) circuit, the tuning methodcomprises: controlling the filter to enter a tuning mode by disablingthe entire filter except for the VCO allowing the VCO to generate anoscillation signal according to a driving signal; comparing a frequencyof the oscillation signal outputted by the VCO with a referencefrequency to generate a comparison result; for adjusting the drivingsignal according to the comparison result in the tuning mode, obtaininga driving signal in the tuning mode when the frequency of theoscillation signal is equal to the reference frequency, and outputtingthe driving signal to the filter in a normal mode; and controlling thefilter to enter the normal mode from the tuning mode by enabling theentire filter to operate according to the driving signal when thefrequency of the oscillation signal is equal to the reference frequency.

Furthermore, a gm replica circuit is disclosed. The gm replica circuitcomprises: a pair of input transistors, each of a first input transistorand a second input transistor of the pair of the input transistorsreceiving a reference voltage, the first input transistor coupled to areference current; a current mirror coupled to the pair of inputtransistors; and a gm setting device, the gm setting device having threeends, a control end of the three ends directly connected to the secondcurrent mirror and the second input transistor, the other two endsrespectively connected to the first input transistor and the secondinput transistor.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a conventional tuning structure.

FIG. 2 is a diagram of another conventional tuning structure.

FIG. 3 is a diagram of another conventional tuning structure.

FIG. 4 is a diagram of a tuning apparatus of a first embodimentaccording to the present disclosure.

FIG. 5 shows a general two-level biquad filter.

FIG. 6 shows a general two-level biquad filter.

FIG. 7 shows an LC ladder filter.

FIG. 8 is a diagram of the entire circuit when the main filter is in thenormal mode.

FIG. 9 is a diagram of the gm replica circuit shown in FIG. 4

FIG. 10 is a diagram of a tuning device of a second embodiment accordingto the present disclosure.

DETAILED DESCRIPTION

In the following filter frequency tuning device and related methodthereof, the tuning procedure is basically divided into two steps. Thefirst step is to use a VCO inside a main filter to adjust the cut-offfrequency of the main filter in order to remove the influence of theprocess variation of the center frequency f_(c). The second step is toutilize a gm replica circuit on-line control to remove the influence ofthe temperature variation of the center frequency f_(c).

The entire implementation of the present disclosure is illustrated asfollows. Please refer to FIG. 4, which is a diagram of a tuningapparatus 400 of a first embodiment according to the present disclosure.As shown in FIG. 4, the tuning apparatus 400 comprises an enablingcircuit 420, a frequency detector (FD) 430, and a controlling circuit440. The controlling circuit 440 comprises a digital-to-analog converter441, and a Gm replica circuit 442. First, when the entire main filter410 is in a tuning mode, a part of the main filter 410 is configured asthe VCO structure 411 by the enabling circuit 420. The presentdisclosure does not limit the options for configuration. For example, aset of trans-conductor cells, which forms a two gm-C integratorstructure, can be set active, and other trans-conductor cells aretemporarily disabled such that the VCO structure 411 can be formed.

A part of a filter can be configured as a VCO circuit. For example, aportion can be selected form a biquad filter or a LC ladder structure toserve as a VCO circuit. Other filter structure (not limited to biquad orLC ladder filter) can become a VCO circuit when a portion of the filterstructure is enabled and other portion is disabled. Please refer to FIG.5 and FIG. 6, which respectively show a general two-level biquad filter.Please also refer to FIG. 7. FIG. 7 shows an LC ladder filter.Basically, a simple VCO circuit can be formed by two gm cells and twocapacitors, where a first capacitor is coupled to the input end of afirst gm cell and the output end of a second gm cell, and a secondcapacitor is coupled to the input end of the second gm cell and theoutput end of the first gm cell. Therefore, as shown in FIG. 5, FIG. 6,and FIG. 7, the highlighted portion can be regarded as theabove-mentioned VCO circuit 411. In addition, the gm-cells, which areutilized to form the VCO, are not limited to be the active gm-cellsinside the filter. Because in a filter, some dummy gm-cells are oftenadded at each node to compensate for the capacitor loading, the dummygm-cells can also be utilized to form the VCO. This change also obeysthe spirit of the present disclosure.

When the VCO structure 411 is formed, the VCO 411 starts to oscillate anoscillation signal having a frequency w_(o)=N*g_(m,u)/C according to adriving signal. At this time, any of the above-mentioned threeconventional mechanisms could be used to compare the frequency f_(o)with the frequency f_(ref). In this embodiment, similar to the digitalcircuit shown in FIG. 3, the digital frequency detector (FD) 430compares the frequency the frequency f_(o) with the frequency f_(ref).The comparing result outputted from the FD 430 is then inputted into thecontrolling circuit 440 to generate the driving signal. The FD 430becomes stable when the frequency f_(o) and the frequency f_(ref) areequal. Please note, because the VCO 411 is inside the main filter 410,when the VCO 411 is completely tuned, theoretically the main filter 410is tuned.

Here, the controlling circuit 440 comprises a DAC 441 and a gm replicacircuit 442. The DAC 441, as mentioned previously, is utilized toconvert the comparing result into a driving signal. In the prior art,the driving signal can be utilized for the main filter 410, however, inthe present disclosure, the gm replica circuit 442 is further utilizedto adjust the driving signal such that the gm value of the gm cell inthe main filter 440 is not influenced by temperature variation.Basically, the gm replica circuit 442 can maintain the gm value byadjusting the driving signal. The operation and function of the gmreplica circuit 442 will be described in the following disclosure.

Therefore, when the VCO 411 is completely tuned, the main filter 410 isconfigured as the original circuit instead of the VCO 411. That is, whenthe frequency of the oscillation signal is equal to the referencefrequency, the enabling circuit 420 switches the entire main filter 410from the tuning mode to a normal mode. At this time, the main filter 410can execute the original function and the main filter 410 is thereafternot utilized as a VCO.

Please refer to FIG. 8, which is a diagram of the entire circuit whenthe main filter 410 is in the normal mode. As shown in FIG. 8, sincethere is no VCO circuit in the normal mode, when the main filter 410 isin the normal mode the FD 430 has no input frequency source to comparewith the reference frequency. In the normal mode, the DAC 441 outputs aconstant analog signal (e.g., a current signal) to the gm replicacircuit 442. The gm replica circuit 442 generates a driving signal toreplicate a gm value to the gm cell in the main filter 410. The gm valueis invariant when environment temperature varies. In other words, thetuned center frequency is temperature insensitive because of the gmreplica circuit 442. The operation and structure of the gm replicacircuit 442 will be discussed in the following disclosure.

Please refer to FIG. 9, which is an exemplary detailed gm replicacircuit. The gm replica circuit 442 comprises a current mirror, an NMOSm1, and a pair of input transistors m2 and m3. A reference voltagedifference ΔV_(ref) is inputted to the gates of the input transistors m2and m3. In addition, the node A and the node B are output nodes of thecurrent mirror. The node A is further connected to the drain of thetransistor m2 and a reference current I_(ref). Please note, in thisembodiment, the reference current I_(ref) is outputted from the DAC 441.The node B is directly connected to the gate of the NMOS m1 andconnected to the drain of the transistor m3. Therefore, the drivingsignal of the node B can be utilized to adjust the resistance of theNMOS m1 such that the gm value of the entire gm replica circuit 442 istuned back to a desired value.

The basic concept of the gm replica circuit 442 is to utilize the innernegative feedback loop to fix the gm value of the gm replica circuit 442as gm=I_(ref)/ΔV_(ref), where I_(ref) is a temperature-insensitivereference current, and ΔV_(ref) is a temperature-insensitive referencevoltage, which can be generated from a bandgap voltage generator.

The following is offered as proof. When the voltage difference ΔV_(ref)is applied to the gates of the transistor m3 and m2, an additionalcurrent Δi is induced to flow through the transistor m3. An additionalcurrent Δi also appears and flows through the transistor m2 toward thenode A. Therefore, the current flowing from the current mirror to thenode A would be (I_(ref)−Δi), where (I_(ref)−Δi) would be equal to Δibecause of the current mirror. Therefore, I_(ref) is equal to 2*Δi.Since gm=2*Δi/ΔV_(ref)=I_(ref)/ΔV_(ref), the resulting gm isI_(ref)/ΔV_(ref), which is invariant when temperature varies. In otherwords, if the Iref and the Vref are both temperature-independent, the Gmvalue of the gm replica circuit is stable and the control voltage usedto control the Gm value can also be utilized in the main filter. Thereference voltage Vref can be generated from a bandgap circuit to ensurethat the reference voltage ΔV_(ref) is not influenced by temperaturevariances. The reference current Iref is received from the DAC.

Moreover, the entire circuit (i.e., the negative feedback) canautomatically adjust the voltage V_(c) to make the gm value alwaysremain fixed at I_(ref)/ΔV_(ref). For example, if the ambienttemperature increases, the gm value increases accordingly. The currentΔi increase because 2*Δi=Gm*ΔV_(ref). Therefore, for the current mirror,the current (I_(ref)−Δi) flowing toward the node A decreases because ofthe increase of the current Δi. And in the node B, the current above thenode B is equal to (I_(ref)−Δi), but the current below the node B isequal to Δi. Therefore, the driving voltage of the node B is pulled downby the current Δi. In other words, the gate voltage of the NMOS mldecreases such that the gm value decreases. Obviously, the entirecircuit is a negative feedback mechanism such that the gm value of theentire gm replica circuit is fixed.

Therefore, as long as the gm cell of the gm replica circuit 442 is equalto the gm cell of the main filter 410, the gm value of the main filter410 can be maintained as that of the gm replica circuit 442 because theyshare the same driving voltage.

In addition, the gm replica circuit 442 is an optional device, and it isnot a limitation of the present disclosure. For example, theimplementation of the gm replica circuit is not limited. For example, ifthe filter can further comprise a negative feedback loop according toits gm cell, the same effect can be achieved. On the other hand, if theentire filter tuning structure includes the gm replica circuit, theoutput of the DAC can be directly utilized to change the I_(ref) orΔV_(ref) in order to further adjust the gm value such that theoscillation frequency of the VCO can be equal to the target value.

Moreover, in some applications, if the temperature variation isinsignificant, or the gm cell is insensitive to the temperaturevariation, the tuning device 400 requires only the DAC 441. This meansthat the gm replica circuit 442 is no longer needed. Please refer toFIG. 10, which is a diagram of a tuning device 1000 of a secondembodiment according to the present disclosure. As shown in FIG. 10, thetuning device 1000 does not comprise a gm replica circuit. This alsoobeys the spirit of the present disclosure.

In addition, in the above disclosure, a digital FD and a DAC are usedfor comparing the oscillation frequency and a reference frequency.Please note, the above-mentioned mechanism is only an embodiment, not alimitation of the present disclosure. For example, the presentdisclosure can also utilize a PLL including an FD, charge pump, and alow pass filter. That is, the FD is utilized to compare the frequencies,and the charge pump and the low pass filter can convert the comparisonresult of the FD into a driving signal for the main filter. This canalso achieve the goal of tuning the center frequency of the main filter.

Please note, in the above disclosure, only the gm value is tuned. But inthe actual implementation, either the gm value or the capacitance can betuned to change the center frequency of the main filter. This changealso obeys the spirit of the present disclosure.

In contrast to the prior art, the present disclosure can utilize thecomponents of the filter to build a VCO and use the VCO to tune thefilter. Therefore, the present disclosure does not need another VCO toperform the tuning operation. This saves the cost and the area that isotherwise required by the VCO. Furthermore, because the presentdisclosure directly utilizes the VCO circuit inside the filter, theenvironment of the VCO and the filter are guaranteed to be identical .Therefore, the mismatch problem between the VCO and the filter, whichmay introduce poor tuning performance, is no longer a concerned. Inother words, the present disclosure has improved tuning performance.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

1. A tuning apparatus for tuning a filter, the filter comprising avoltage-controlled oscillator (VCO) circuit, the tuning apparatuscomprising: an enabling circuit, electrically connected to the filter,for controlling the filter to enter a tuning mode by disabling theentire filter except for the VCO and allowing the VCO to generate anoscillation signal according to a driving signal; and for controllingthe filter to enter a normal mode from the tuning mode by enabling theentire filter to operate according to the driving signal when afrequency of the oscillation signal is equal to a reference frequency; afrequency detector, for comparing a frequency of the oscillation signaloutputted by the VCO with the reference frequency to generate acomparison result; and a controlling circuit, electrically connected tothe frequency detector and the filter, for adjusting the driving signalaccording to the comparison result in the tuning mode, and outputtingthe driving signal to the filter in the normal mode.
 2. The tuningapparatus of claim 1, wherein the filter is a transconductor-capacitor(Gm-C) filter comprising a plurality of capacitors and gm cells.
 3. Thetuning apparatus of claim 2, wherein the driving signal is capable ofchanging either transconductance of the gm cells or capacitance of thecapacitors.
 4. The tuning apparatus of claim 2, wherein the VCOcomprises: a first gm cell having a first input end and a first outputend; a second gm cell having a second input end and a second output end;a first capacitor, electrically connected to the first input end and thesecond output end; and a second capacitor, electrically connected to thesecond input end and the first output end.
 5. The tuning apparatus ofclaim 4, wherein the driving signal is capable of changing eithertransconductance of the first and the second gm cells or capacitance ofthe first and the second capacitors.
 6. The tuning apparatus of claim 1,wherein the frequency detector is a digital frequency detector, and thecontrolling circuit comprises a digital-to-analog converter (DAC) forconverting the comparison result into the driving signal.
 7. The tuningapparatus of claim 6, wherein the controlling circuit further comprisesa gm replica circuit, coupled to the DAC, for adjusting the drivingsignal in the normal mode according to ambient temperature variation. 8.The tuning apparatus of claim 7, wherein the gm replica circuitcomprises: a pair of input transistors, each of a first input transistorand a second input transistor of the pair of the input transistorsreceiving a reference voltage, the first input transistor coupled to areference current outputted from the DAC; a current mirror coupled tothe pair of input transistors; and a gm setting device, the gm settingdevice having three ends, a control end of the three ends directlyconnected to the second current mirror and the second input transistor,the other two ends respectively connected to the first input transistorand the second input transistor.
 9. The tuning apparatus of claim 8,wherein the current mirror comprises a first current output node and asecond current output node, the first input transistor and the secondinput transistor are MOS transistors, the first input transistorcomprises: a gate, coupled to a first reference voltage; a drain,coupled to the first output current node and the reference current; anda source, coupled to the gm setting device; and the second inputtransistor comprises: a gate, coupled to a second reference voltage; adrain, coupled to the second current output node and directly connectedto the control end of the gm setting device; and a source, coupled tothe gm setting device.
 10. The gm replica circuit of claim 9, whereinthe gm setting device comprises a MOS transistor, where a gate of theMOS transistor is the control end.
 11. The tuning apparatus of claim 1,wherein the controlling circuit comprises a converter for converting thecomparison result into the driving signal.
 12. The tuning apparatus ofclaim 11, wherein the controlling circuit further comprises a gm replicacircuit, coupled to the converter, for adjusting the driving signal inthe normal mode according to ambient temperature variation.
 13. Thetuning apparatus of claim 12, wherein the gm replica circuit comprises:a pair of input transistors, each of a first input transistor and asecond input transistor of the pair of the input transistors receiving areference voltage, the first input transistor coupled to a referencecurrent from the converter; a current mirror coupled to the pair ofinput transistors; and a gm setting device, the gm setting device havingthree ends, a control end of the three ends directly connected to thesecond current mirror and the second input transistor, the other twoends respectively connected to the first input transistor and the secondinput transistor.
 14. The tuning apparatus of claim 13, wherein thecurrent mirror comprises a first current output node and a secondcurrent output node, the first input transistor and the second inputtransistor are MOS transistors, the first input transistor comprises: agate, coupled to a first reference voltage; a drain, coupled to thefirst output current node and the reference current; and a source,coupled to the gm setting device; and the second input transistorcomprises: a gate, coupled to a second reference voltage; a drain,coupled to the second current output node and directly connected to thecontrol end of the gm setting device; and a source, coupled to the gmsetting device.
 15. The gm replica circuit of claim 14, wherein the gmsetting device comprises a MOS transistor, where a gate of the MOStransistor is the control end.
 16. A gm replica circuit comprising: apair of input transistors comprising a first input transistor and asecond input transistor, the pair of input transistors receiving areference voltage, the first input transistor coupled to a referencecurrent; a current mirror coupled to the pair of input transistors; anda gm setting device, the gm setting device having three ends, a controlend of the three ends coupled to the second input transistor, the othertwo ends respectively connected to the first transistor and the secondinput transistor.
 17. The gm replica circuit of claim 16, wherein thecurrent mirror comprises a first current output node and a secondcurrent output node, the first input transistor and the second inputtransistor are MOS transistor, the first input transistor comprises: agate, coupled to a first reference voltage; a drain, coupled to thefirst output current node and the reference current; and a source,coupled to the gm setting device; and the second input transistorcomprises: a gate, coupled to a second reference voltage; a drain,coupled to the second current output node and directly connected to thecontrol end of the gm setting device; and a source, coupled to the gmsetting device.
 18. The gm replica circuit of claim 17, wherein the gmsetting device comprises a MOS transistor, where a gate of the MOStransistor is the control end.
 19. A tuning method for tuning a filter,the filter comprising a voltage-controlled oscillator (VCO) circuit, thetuning method comprising: controlling the filter to enter a tuning modeby disabling the entire filter except for the VCO and allowing the VCOto generate an oscillation signal according to a driving signal;comparing a frequency of the oscillation signal outputted by the VCOwith a reference frequency to generate a comparison result; adjustingthe driving signal according to the comparison result in the tuningmode; and controlling the filter to enter a normal mode from the tuningmode by enabling the entire filter to operate according to the drivingsignal when the frequency of the oscillation signal is substantiallyequal to the reference frequency.
 20. The tuning method of claim 19,further comprising: providing a gm replica circuit; and utilizing the gmreplica circuit to adjust the driving signal in the normal modeaccording to ambient temperature variation.
 21. The tuning apparatus ofclaim 19, wherein the filter is a transconductor-capacitor (Gm-C) filtercomprising a plurality of capacitors and gm cells, and the drivingsignal is capable of changing either transconductance of the gm cells orcapacitance of the capacitors.